1. Field of the Invention
The invention relates generally to systems and methods for operation of a milling bottom hole assembly which is incorporated into a coiled tubing running string.
2. Description of the Related Art
A significant number of wells in shale formations require hydraulic fracturing stimulations for economical production. The vast majority of these stimulations are performed by simultaneously fracturing several clusters of perforations followed by a mechanical isolation with composite bridge plugs. The number of plugs is directly dependent on the stimulated well length. Thus, between 20 and 100 plugs are generally used for each well.
After the bridge plugs have been set, they will later need to be removed. Plugs near the entrance to the wellbore can typically be removed with a positive displacement motor and mill run on jointed pipe. Removal of plugs further away from the entrance to the wellbore, either in vertical wells or in long laterals, becomes more challenging as the efficiency of the end loads transmitted from surface to the mill are reduced. This efficiency reduction is primarily due to helical buckling in the coiled tubing near the mill. According to studies available in public literature, the buckled section of coiled tubing might be as long as 20% of the total length of the coiled tubing running string. Because of this helical buckling, quite often the deepest plugs cannot be reached with standard coiled tubing arrangements. Even if the deepest plugs can be reached, helical buckling can render the mill inefficient or unable to mill properly and remove the plug.
A related problem for these types of milling operations is power management. Downhole power is needed to operate bottom hole assembly (BHA) sensors and tools. However, there are operational difficulties. Large downhole batteries are impractical due to their size. Supplying operating power from the surface is also problematic. A conductive cable that is able to transmit a significant amount of power to a downhole tool has a larger diameter. Flow area within a coiled tubing running string would be restricted by a large cable within the coiled tubing flowbore. Additionally, the increase in weight associated with the larger conductive wire is problematic.